1. Field of the Invention
The present invention relates to control systems for variable reluctance electrical motors and in particular to control systems for drive systems such as those forming the subjects of our co-pending patent application Ser. Nos. 789,038 and 789,095, respectively "Variable Speed Variable Reluctance Electric Machines" and "Servomotor Control Systems" now U.S. Pat. Nos. 4,670,696 and 4,661,756. The drive systems described in said co-pending Applications incorporate a variety of control features for association with variable reluctance electrical motors. These control features are also useful in themselves in selfcontained control systems not necessarily incorporating a variable reluctance electrical motor, but applicable to a variety of such motors.
The invention also relates to test and set-up methods for drive systems for variable reluctance motors and in particular to such methods when applied to drive systems in accordance with the features disclosed in our co-pending patent application Ser. No. 789,038 entitled "Variable Speed Variable Reluctance Electrical Machines" and to drive systems incorporating control systems in accordance with the present invention.
2. Brief Summary of the Invention
According to a first aspect of the invention, there is provided a control system for a variable reluctance electrical motor, comprising means for connecting a voltage source or sources across driving member windings of a said motor, said windings being connectible across said source or a said source in a predetermined sequence and for a predetermined increment of motor driven member displacement, and means for regulating the instantaneous magnitude of the current in a driving member winding when connected to said source or a said source, said current-regulating means being responsive to a driven member position dependent signal of driven-member position-sensing means of or associated with said motor to regulate said current magnitude so that the instantaneous value of said current set by said regulating means at any position of the driven member within said increment of driven member displacement during which the winding is connectible to said source or a said source relative to its any value at any other said position is substantially determined by the instantaneous position of the driven member.
The control system according to the invention may comprise means for connecting a voltage source or sources across stator pole windings of a said motor, said windings being connectible across said source or a said source in a predetermined sequence during rotation of the rotor of the motor and each stator pole winding being thus connectible for a predetermined angular increment of rotor rotation, and means for regulating the instantaneous magnitude of the current in a stator winding when connected to said source or a said source, said currentregulating means being responsive to a rotor position-dependent signal of rotor position-sensing means associated with said motor to regulate said current magnitude so that the instantaneous value of said current set by said regulating means at any angular position of the rotor within said increment of rotor rotation during which the winding is connectible to said source or a said source relative to its value at any other said angular position is substantially determined by the instantaneous angular position of the rotor.
The current-regulating means may be responsive to said rotorposition dependent signal to regulate said current magnitude so that successive instantaneous values of said current during an initial portion of said angular increment of rotor rotation during which the winding is connectible to said source or a said source increase progressively with progressive rotation of the rotor and successive instantaneous values of said current during a terminal portion of said angular increment decrease progressively with said progressive rotation.
The current-regulating means may be responsive to said rotorposition dependent signal to regulate said current magnitude so that the rate at which successive instantaneous values of said current decrease during said terminal portion of said angular increment of rotor rotation is substantially the same as the rate of increase of successive instantaneous current values during said initial portion and the succession of instantaneous current values over said angular increment of rotor rotation substantially defines a substantially symmetrical current waveshape extending over said angular increment.
The current-regulating means may also be responsive to said rotor-position dependent signal to regulate said current magnitude so that successive instantaneous values of said current during said initial portion of said angular increment of rotor rotation substantially define the rising current region of a substantially sinusoidal current halfwave and successive instantaneous values of said current during said terminal portion of said angular increment substantially define the falling current region of a substantially sinusoidal current halfwave An intermediate portion of said angular increment of rotor rotation may optionally be interposed between said initial and terminal portions and the current magnitude may remain substantially constant during said intermediate portion of said angular increment of rotor rotation. In an especially preferred embodiment of the control system according to the invention, said current-regulating means is responsive to said rotor-position dependent signal to regulate said current magnitude so that said instantaneous current values during said angular increment of rotor rotation substantially define a substantially sinusoidal halfwave.
The control system according to the invention may further comprise means for producing a signal, the value of which is indicative of a desired parameter of motor operation, said current-regulating means also being responsive to said parameter-indicative signal to regulate said stator winding current so that the absolute magnitude of said current at every angular position of the rotor within said angular increment of rotor rotation during which the winding is connectible to said voltage source or a said voltage source is substantially determined by the value of said Parameter-indicative signal.
Control systems according to the invention applied to reluctance motor drives permit exceptionally quiet operation of the motor to be achieved, which represents a significant advantage in the commercial and industrial application of these motors. As noted in our co-pending Applications, reluctance motor power drives have always been noteworthy for the amount of noise they generate. By cutting down on their noise, the acceptability of reluctance motors to users is accordingly enhanced.
By controlling the relative instantaneous magnitude of the current input to typically each pair of stator phase windings of a machine, as they are sequentially energised in accordance with the angular position of the rotor within the appropriate angular segment of rotor rotation, the torque developed by each phase of the machine during its period of energisation may be closely controlled so that the detrimental effects of rapid torque rise at the start of pole overlap and torque ripple where the torques of successive phases overlap may be minimised. Regulation of current in the outgoing phase is related to the rate of current rise in the incoming phase to give a substantially constant nett machine torque output. This smooth transition may be achieved with substantially any construction of the machine poles, although as set out in a co-pending patent application, pole shaping is also of benefit in implementing noise-free and vibration-free operation of reluctance motors. In the particularly favoured constructions of the present invention as disclosed in the present patent Application, smooth sinusoidal variations in phase currents are employed with especially satisfactory results in regard to noise and vibration. The high level of disturbing noise and vibration so characteristic of many prior art reluctance motor drives may be largely eliminated by use of the control system of the invention, both with and without complementary pole shaping techniques.
Quiet operation is especially associated with energisation of the phases by means of sinusoidal current waveforms, but is in no way limited to this embodiment of the control system of the invention. Ramped wave forms may also be employed, the initial and terminal rates of current rise and fall being selected by methods such as those forming the subject of a further aspect of the present invention, hereinafter described. In addition, the entire waveform profile may be tailored to suit each individual motor phase to bring about smooth transfer of torque between phases during operation in accordance with said methods and such tailoring may be incorporated in the production process for the control systems for the motors, so that each motor's performance may be optimised in regard to both torque smoothness and quietness of operation.
Thus in order to improve machine performance in terms of noise and vibration, motor torque may be controlled by current regulation, as also described in said co-pending Applications, to minimise the detrimental effects of rapid torque rise at the start of pole overlap and torque ripple where the torques of successive phases overlap. In particular, current in an outgoing phase may be regulated so that the torque of that phase is reduced at a rate such that the torque developed by the outgoing phase taken together with the torque being developed by the incoming phase together amounts to a substantially constant net machine torque output.
It is a further object of the present invention to provide a method for enabling appropriate values of phase winding or exciting current to be established for each disposition of the rotor or driven member of a variable reluctance motor relative to the driving member or stator at least during a transition between phases, this disposition being typically an angular orientation of a rotary driven member or rotor.
According therefore to a further aspect of the invention, there is provided a method of establishing relationships between a plurality of parameters of operation of a saturable variable reluctance electrical machine, said electrical machine comprising a stationary or driving member having a plurality of salient driving poles, a magnetising winding for each driving pole, a movable or driven member having a plurality of salient driven poles, the number of driven poles being less than the number of driving poles, the airgap between each driving pole and a driven pole positioned in alignment therewith being small relative to the dimensions of the poles transverse to said airgap so that in operation of the machine magnetic saturation occurs in the region of the mechanically variable interface or overlap between the driving and driven poles, and the extents and dispositions of the driven poles being related to those of the driving poles so that in operation of the machine the force-producing increment of driven member displacement resulting from the mechanical interface or overlap of each driven pole with a driving pole overlaps the force-producing increment of driven member displacement resulting from the overlap of another driven pole with a further driving pole. said parameters including an exciting current for at least one said magnetising winding, the disposition of the movable or driven member relative to the stationary, or driving member, and the force exerted on the driven member in the presence of a said exciting current in one or more of said magnetising windings, wherein a measured level of exciting current is applied to one of said magnetising windings while the position of the driven member is maintained fixed relative to the driving member, and the force exerted on the driven member during the application of said exciting current to said winding is determined.
In the application of the method to a rotary machine comprising a stator having a plurality of salient stator poles, a magnetising winding for each stator pole, a rotor having a plurality of salient rotor poles, the number of rotor poles being less than the number of stator poles, the radial airgap between each stator pole and a rotor pole positioned in alignment therewith being small relative to the dimensions of the poles transverse to said airgap so that in operation of the machine magnetic saturation occurs in the region of the mechanically variable interface or overlap between the stator and rotor poles, and the arcuate extents and dispositions of the rotor poles being related to those of the stator poles so that in operation of the machine the torque-producing angular increment of rotor rotation resulting from the mechanical interface or overlap of each rotor pole with a stator pole overlaps the torque producing angular increment of rotor rotation resulting from the overlap of another rotor pole with a further stator pole, said parameters include an exciting current for at least one said magnetising winding, the disposition of the rotor relative to the stator, and the torque exerted on the rotor in the presence of a said exciting current in one or more of said magnetising windings, a measured level of exciting current is applied to one of said magnetising windings while the position of the rotor is maintained fixed relative to the stator, and the torque exerted on the rotor during the application of said exciting current to said winding is determined.
In order to derive a further useful relationship between said parameters, an exciting current may also be applied to a further said magnetising winding and this further current may be varied in magnitude until said force or torque exerted on the driven member or rotor has a predetermined value.
Apparatus for use in the method of the invention preferably comprises means for applying a respective exciting current to one or more of said magnetising windings, means for controlling the magnitude of the or each said exciting current, means for maintaining the driven member or rotor in a predetermined position relative to the driving member or stator, and means for measuring the force exerted on the driven member or the torque exerted on the rotor when at least one of the magnetising windings is energised.
Accordingly, the torque contribution of a particular stator phase may be measured in a static test in which the phase winding is energised with DC current. However interaction between phases when two or more windings are energised simultaneously means that each individual phase to phase transition may require to be experimentally determined with both phases energised. Accordingly in such testing, one phase current waveform may be arbitrarily selected and the other phase current variation then tailored to give torque smoothness, involving a static test according to the present invention in which the rotor is attached to a torque-measuring device and locked in a series of positions in the transition region. In each rotor position an appropriate value of the selected current is established in one phase as a DC current and the DC current in the other phase is adjusted to give the required constant torque reading. Repetition of this procedure at intervals over the torque overlap region defines the current waveforms required for torque smoothness. These experimentally-determined current waveforms are then approximated by digitised wave-forms with step variations in current at 0.5.degree. intervals to give the values needed, for example, for an EPROM based system, or they are incorporated in an analogue construction of the current-regulating means.